7.1 Summary

Concentrations of DOC have been measured reliably in groundwater since the early 1970s. Since then, these measurements have been made for a wide variety of aquifer systems for a variety of scientific purposes. Several investigators working in arid environments in the western United States, for example, discovered that DOC transported by shallow groundwater to seasonal streams was an important contributor to the structure of local ecosystems (Hornberger et al., 1994; Findlay and Sobczak, 1996; Boyer et al., 1997; Baker et al., 2000). Other investigators viewed DOC as an important contributor to observed changes in groundwater chemistry as it flowed downgradient in regional aquifers (McMahon and Chapelle, 1991a), and how it contributes to aquifer diagenesis (McMahon et al., 1992). When the bioremediation of anthropogenic contaminants became an important environmental issue in the 1990s, DOC began to be considered as a competing substrate, negatively affecting the biodegradation of petroleum hydrocarbons but positively affecting the biodegradation of chlorinated solvents (Wiedemeier et al., 1999).

Because of this variety of issues and approaches, a consensus concerning the overall hydrologic, ecologic, and geochemical significance of DOC in groundwater has been slow to emerge. Beginning with studies of DOC dynamics in soils and groundwater (Cronan and Aiken, 1985; Jardine et al., 1989), it gradually became clear that DOC was adsorbing rapidly and reversibly with soil components such as ferric hydroxides and POC. This provided an explanation for DOC’s observed behavior in groundwater, including its rapid removal and remobilization depending on hydrologic conditions. In addition, these adsorption processes explained the ubiquitous low concentrations of DOC in groundwater not directly impacted by surface water sources that had been observed early on (Leenheer et al., 1974).

All of this led to the view, first expressed by Aiken (1989), that groundwater systems behave like naturally occurring chromatographic columns in which components of DOC are systematically separated depending on their chemical and physical properties (Shen et al., 2015; Figure 8). Like chromatographic columns, these adsorption processes are partially reversible and there is a continuous interplay between the adsorbed and dissolved compartments. Unlike chromatographic columns, however, a variety of microbially-mediated redox processes are superimposed on these sorption-desorption processes. Those redox processes oxidize DOC, POC, and AOC with the sequential reduction of dissolved oxygen, ferric iron, and sulfate, and carbon dioxide. The net result of these redox processes is to increase concentrations of dissolved solids in groundwater as it flows along the hydrologic gradient while simultaneously decreasing the bioavailability of the remaining DOC, POC., and AOC.

The literature reviewed in this book suggests that the composition and bioavailability of DOC in groundwater reflect the bioavailability of the associated POC and AOC compartments as well. The chief hydrologic and geochemical significance of DOC may be that it indicates the bioavailability, and thus the reducing capacity, of total organic carbon present in aquifer systems. That reducing capacity, which determines the kinds of reduction/oxidation processes that can occur, is a principal driver for the geochemical processes that determine groundwater quality in both pristine and human-impacted aquifer systems.


Dissolved Organic Carbon in Groundwater Systems Copyright © 2022 by Francis H. Chapelle. All Rights Reserved.